FIG. 10 depicts an arrangement of a wireless base station in a mobile communication system as a related art.
This wireless base (BS) station 10 is schematically composed of a transmitting/receiving portion 100 connected to an antenna ANT, a baseband signal processor 200 performing a baseband signal processing to a signal transmitted/received or transferred between a mobile station (MS) 20 and the wireless base station 10, and a communication network interface (IF) 300 serving as an interface with a mobile communication network NW.
In such a mobile communication system, due to a Doppler effect caused by a movement of the mobile station 20, the wireless base station 10 has a frequency shift (variation) arisen in an uplink signal received from the mobile station 20.
Therefore, the baseband signal processor 200 within the wireless base station 10 is provided with an AFC (Automatic Frequency Control) function for automatically controlling the frequency of an uplink signal received in accordance with a specified allocation frequency (see e.g. Japanese Laid-open Patent Publication No. 2005-295282).
In the above related art, the wireless base station provided in such a place that a mobile station moves at a high speed is desired to have a high AFC capability of dealing with a large frequency variation, which causes a problem that the circuit scale within the wireless base station is to be increased.
Hereinafter, the above problem will be described by taking as an example a case where the mobile station 20 moves through positions P1 to P2 to P3 while passing through the wireless base station 10 as depicted in FIGS. 11A-11C, where it is supposed that a user (not depicted) holding the mobile station 20 boards a high speed train TR moving at a speed of V=“250 Km/h”.
As depicted in FIG. 11A, the mobile station 20 receives a downlink signal DS, where an allocated frequency FD=“2.10 GHz”, transmitted from the wireless base station 10 at the position P1. Now supposing that the position P1 be sufficiently distant from the antenna ANT of the wireless base station 10 so that an angle θ, formed between the travelling direction of the mobile station 20 and the arrival direction of the downlink signal DS, is almost zero (θ “0”), a reception frequency (hereinafter, mobile station reception frequency) RF20 of the downlink signal DS in the mobile station 20 assumes a value given by the following Eq. (1):MS reception freq. RF20=downlink allocated freq. FD(2.10 GHz)+downlink Doppler shift freq. DD(495 Hz)  Eq. (1)
It is to be noted that the downlink Doppler shift frequency DD=“495 Hz” in the above Eq. (1) is calculated according to the following Eq. (2), where an uplink Doppler shift frequency as will be described below is similarly calculated by Eq. (2):Down link Doppler shift freq. DD=moving speed V*downlink allocated freq. FD/light speed C  Eq. (2)
Then, the mobile station 20 transmits to the wireless base station 10 to the wireless base station 10 an uplink signal US, where allocated frequency FU=“1.90 GHz”, at a transmission frequency TF given by the following Eq. (3):MS transmission freq. TF=uplink allocated freq. FU(1.90 GHz)+downlink Doppler shift freq. DD  Eq. (3)
Namely, for producing the transmission frequency TF in synchronization with the reception frequency RF20, the mobile station 20 transfers the downlink Doppler shift frequency DD to the uplink signal US without changes. Accordingly, the reception frequency (hereinafter, occasionally BS reception frequency) RF10 of the uplink signal US in the wireless base station 10 is then to assume a value given by the following Eq. (4) under the influence of the uplink Doppler shift frequency Du (451 Hz):
                                                                        BS                ⁢                                                                  ⁢                reception                ⁢                                                                  ⁢                                  freq                  .                                                                          ⁢                                      RF                    10                                                              =                            ⁢                                                MS                  ⁢                                                                          ⁢                  transmission                  ⁢                                                                          ⁢                                      freq                    .                                                                                  ⁢                    T                                    ⁢                                                                          ⁢                  F                                +                                                                                                      ⁢                              uplink                ⁢                                                                  ⁢                Doppler                ⁢                                                                  ⁢                shift                ⁢                                                                  ⁢                                  freq                  .                                            ⁢                                                                                                                                    ⁢                                                D                  U                                ⁢                                                                  ⁢                                  (                                      451                    ⁢                                                                                  ⁢                    Hz                                    )                                                                                                        =                            ⁢                                                F                  U                                +                                  (                                                            D                      D                                        +                                          D                      U                                                        )                                                                                                        =                            ⁢                                                1.90                  ⁢                                                                          ⁢                  GHz                                +                                  946                  ⁢                                                                          ⁢                  Hz                                                                                        Eq        .                                  ⁢                  (          4          )                    
Namely, in the uplink signal US received by the wireless base station 10, both of the downlink Doppler shift frequency DD and the uplink Doppler shift frequency Du appear as a frequency variation.
Then, as the mobile station 20 approaches the antenna ANT of the wireless base station 10, frequency variation ranges of the base station reception frequency RF10 and the MS reception frequency RF20 present characteristics decreasing from “946 Hz” and “495 Hz” respectively as depicted by characters CR10 and CR20 in FIG. 12. This is because as the above angle θ increases, the BS direction component (V cos θ) of the moving speed V of the mobile station 20 decreases so that the downlink Doppler shift frequency DD and the uplink Doppler shift frequency Du both decrease.
When the mobile station 20 arrives at the position P2 as depicted in FIG. 11B, the BS direction component of the speed V assumes “0”, so that no Doppler shift is caused in the downlink signal DS and the uplink signal US, whereby the BS reception frequency RF10 and the MS reception frequency RF20 become equal to the uplink allocated frequency Fu and the downlink allocated frequency FD, respectively.
As the mobile station 20 moves away from the antenna ANT, the BS reception frequency variation characteristic CR10 and the MS reception frequency variation characteristic CR20 present negative characteristics further decreasing from “0 Hz” as depicted in FIG. 12. Namely, when the mobile station 20 moves away from the wireless base station 10, a negative Doppler shift is caused in both of the uplink and downlink directions.
When the mobile station 20 arrives at the position P3 as depicted in FIG. 11C, the MS reception frequency RF20 and the BS reception frequency RF10 assume respective values given by the following Eqs. (5) and (6), where it is supposed that the position P3 be sufficiently distant from the antenna ANT like the position P1, i.e. the above angle θ “0” is established:MS reception freq. RF20=downlink allocated freq. FD downlink Doppler shift freq. DD  Eq. (5)
                                                                        BS                ⁢                                                                  ⁢                reception                ⁢                                                                  ⁢                                  freq                  .                                                                          ⁢                                      RF                    10                                                              =                            ⁢                              MS                ⁢                                                                  ⁢                transmission                ⁢                                                                  ⁢                                  freq                  .                                                                          ⁢                  T                                ⁢                                                                  ⁢                F                                                                                                      ⁢                              uplink                ⁢                                                                  ⁢                Doppler                ⁢                                                                  ⁢                shift                ⁢                                                                  ⁢                                  freq                  .                                                                                                                      ⁢                              D                U                                                                                        =                            ⁢                                                F                  U                                ⁡                                  (                                                            D                      D                                        +                                          D                      U                                                        )                                                                                                        =                            ⁢                              1.90                ⁢                                                                  ⁢                GHz                ⁢                                                                  ⁢                946                ⁢                                                                  ⁢                Hz                                                                        Eq        .                                  ⁢                  (          6          )                    
Namely, when being set under the condition depicted in FIGS. 11A-11C, the wireless base station 10 is desired to have a very high AFC capability of dealing with a frequency variation range of “946 Hz” to “+946 Hz”. Also, as the moving speed V of the mobile station 20 is increased, the influence of the Doppler shift is increased, so that the wireless base station 10 is desired to have an AFC capability of dealing therewith.
If the AFC capability of the wireless base station 10 is low, a problem depicted in FIGS. 13A-13C will arise, where it is supposed that the wireless base station 10 manage two sectors SCT1 and SCT2 as depicted in FIG. 13A and the mobile station 20 perform a transmission/reception of the downlink signal DS and the uplink signal US with the sector SCT1.
In this state, when the mobile station 20 moves away from the sector SCT1 as depicted in FIG. 13B, it becomes possible to perform a transmission/reception with the sector SCT2, so that the mobile station 20 transmits to the sector SCT1 a switchover request REQ toward the sector SCT2 (or additional request to the sector SCT2).
However, if the AFC capability of the wireless base station 10 is low, the sector switchover request REQ is not normally processed or received, so that when the mobile station 20 moves further away from the sector SCT1 as depicted in FIG. 13C, the mobile station 20 may not perform a transmission/reception in any of the sectors SCT1 and SCT2, causing a communication disconnection.